6 Month Gauss Cannon Project / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

6 Month Gauss Cannon Project

first previous 4 5 6 7 next last

Move Thread

LAN_403

guitardude012

Sat Jan 03 2009, 05:24AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

Are you saying have a photocell(photoresistor) as opposed to photo diode? This is actually the slowest of the three. Have you watched the rise/fall time on a scope?

For a barrel try polycarbonate tubing. It is relatively cheap and has the benefit of being clear (good for optical trigger relocation). It is also available in our usual diameters. Carbon fiber tubing is easily crushed, it is better when tensile strength is required.

blackgrunge

Sat Jan 03 2009, 06:41AM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

@guitardude012

nooo...if you read what I said carefully you would see that I'm using a photo diode has the same properties as a photo resistor if you know what your talking about. A photo diode acts just like a transistor but the base is the light sensor and the collector and emitter are the positive and negative leads. No matter which one you call it, they still both do the exact same thing. A photo cell, which is what you were referring to, operates completely differently. Do some homework before you start attacking my setup. I've put my entire setup across a digital oscilloscope at the local university I know what my rise and fall times are. Your missing the fact that I'm using a inverting schmitt trigger to handle my I/O for my optics. This allows me to physically control the response time for all of my optical sensors by simply adjusting the resistance pulling the input high or low. It also allows me to control how sensitive my sensors are to ambient lighting. Being yet another reason why I'm using true IR instead of red light from some laser pointer's laser diode. As far as clear tubing is concerned it refracts all sorts of light because of the grain of the plastic. And as far as carbon fiber being too brittle, I'm using arrow shafts. Your really going to tell me that arrow shafts are more brittle than the alternatives? They have far more flexibility and tensile strength than any other material that has with a wall thickness of 1.2mm . I appreciate the input but try not to bash my project when you don't have your facts straight. I'm going to look into polycarbonate as an alternative but I'm still going to slot it even if I decide to.

@rp181

Yah I think i'm going to call up grandpower and see if they'll ship me a few of the 50RIA80's they are about $12 brand new so i'll see if they wouldn't mind donating 5-6 of them. If push comes to shove, I have enough of the 1200v to put in parallel but that requires too much work on my part...I'm kinda lazy about that kind of stuff.

That's what I'm using is carbon fiber, but they are carbon fiber arrow shafts aka graphite, not true carbon fiber.

Oh and just to bounce some ideas off people. I just completely rewired all my circuitry, except my logic gate setup. Howare people mounting their SCRs and what to? I've done some design sketches for heat sinks when I need them CNCed but I still don't know how to mount them in the mean time and keep them insulated. Right now their mounted (as you can see in the picture of my original post) to square nylon rod but I fear that when I go to hook up my big capacitor that it will melt the nylon. I'm going to take some pictures of the new setup with the redesigned tomorrow morning (because its about 1:40am right now and I'm tired). I'd just like to hear some suggestions on how to 'clean up' all the wiring I have. I've got some terminal strips lying around and they are a pain in the ass to use and I also have (as you'll see in the picture) these serial ports that I've started to use to keep all my stuff organized. I just need some cosmetic suggestions because I've got wires going all over the place and I don't want the wrong ones to start getting crossed.

guitardude012

Sat Jan 03 2009, 08:27AM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

I am sorry if I came off as hostile, I'm working on two other projects at the moment and was trying to leave brief responses that may have been of use.

I was only saying that carbon fiber was much stronger for tensile strength, which isn't what is required for this application. I never said graphite was brittle(or even carbon fiber for that matter), as I said nothing of yield strength, but that the compressive strength of carbon fiber tubing isn't its main advantage. I'm really not worried about crushing the barrel permanently, but there is a good chance the barrel is or will be squeezed against the projectile which is hard to detect.

As for infrared light helping avoid ambient light, that depends on how dirty your lightsource is, but I doubt even the sun would cause problems if your photodiodes are shielded.

You never really said which sensors you had, and your comments were misleading because although photodiodes are available in a type that operates in photoconductive mode, saying that they have to "get above 300k ohms to pull it high" is really the wrong way to be thinking about them. Thank you for telling me to get my facts straight even though I suggested the same solution you had chosen, and you had misguided my response by putting resistance and photodiode in the same sentence.

I'm not sure exactly what you are trying to accomplish in terms of your SCR mounting question so I won't offer a specific solution again because you will most likely attack me, but aluminum can be a cheap and locally available alternative to copper until you get your design completed.

I didn't once attack your personal knowledge but I appreciate you questioning mine. Your initial description of your photogates left a lot of room for misinterpretation.

rp181

Sat Jan 03 2009, 04:52PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Carbon fiber has great compressive strength. Just take a look at mcmaster-carr, they give that kind of data. The compressive strength of one i was looking at was 100,000PSI, thats alot. Take a look at these:

Heatsinks are utterly pointless. The SCR's will not get above barely warm. I am saying this from experience, plus some engineers over at chtechnology confirmed this.

Those SCR's aren't very big, you would be better off with larger ones. If you are going to but them, you can get them at digikey:

EDIT:Just kidding, min. quantity is 100.....
Also, i think grandpower was replaced with chtechnology. If you look at chtechnology, they are the same, some of there contacts are even "@grandpower.com". Talk to Darrick Schiebe at chtechnology.

Make sure all you wires connecting to the capacitor bank are securely fastened, when fired, repulsive forces cause wire to fly and yank connections.
I agree with the above post regarding aluminum. It can be bought at homedepot.
After i had problems with oxidation, i used flattened copper pipe. It came out quite well:

guitardude012

Sat Jan 03 2009, 09:20PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

rp181 wrote ...

Heatsinks are utterly pointless. The SCR's will not get above barely warm. I am saying this from experience, plus some engineers over at chtechnology confirmed this.

I agree, in fact even the small built in heatsinks don't help much for pulses in the few ms range. Heatsinks will matter if he plans to fire it rapidly enough... but I would start worrying about melting wire enamel before needing SCR heatsinks.

How conductive are arrow shafts? I know from wiring race cars that carbon fiber(especially the epoxy resins) are very conductive.

rp181

Sat Jan 03 2009, 09:37PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

That shouldn't matter. The wire has insulation, people use brass as barrels.
The only time i had a large stud SCR get warm was when it was malfunctioning. Current was passing through with no gate signal, like a resistor. It seems that the internal resistor (used to help prevent false triggerings from the gate) had blown and shorted.

guitardude012

Sat Jan 03 2009, 09:40PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

I was only worried about the barrel conducting HV if the enamel became scratched or worn due to high temperatures.

A few from this forum have even managed to shatter coils unintentionally.

blackgrunge

Sat Jan 03 2009, 09:50PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
Location:
Posts: 55

He's referring to eddy currents concering barrel conductivity. Once I return home I will measure the resistivity per foot of the graphite barrel. The enamel thats used on the wire is bilayer. It has a heat resistant paint covered by clear enamel. Its operating temperature is near 150 degrees Celsius. I'm also in the process of having coil heat sinks manufactered that will using an osmium oxide paste to maximize surface area contact with the heat sink.

My coils are insulated from the barrel by a very thin tube that sits between the coil and the barrel allowing it to slide freely if adjustment is necessary. It's also secured in place during firing so it won't move.

With the 5kJ model built it will have a firing time (so my calculations say) of around one shot every 7 seconds. That's assuming that I can make it through more than 3 shots without losing an SCR or two. The coils themselves might not be able to handle the stress of such a demand because the resistivity of the copper will spike and the heat sink may not be able to cool it quick enough.

Using a brass barrel is reckless. A prime example of considerations that need to be made. Using a conductor as a barrel will create more eddy currents than the projectile will. I chose graphite because it conducts fairly poorly but I will back that with some hard numbers once I measure the resistivity.

Did anyone come up with any ways to clean up my wiring setup or did anyone have any suggestions? I still need to take the pics of how I improved the wiring a few nights ago.

guitardude012

Sat Jan 03 2009, 10:01PM

Registered Member #968 Joined: Fri Aug 24 2007, 04:54PM
Location: Norman, Oklahoma
Posts: 104

I may have missed it... how many coils are you planning to split the 5kJ between? If it is 4 or more I doubt you will have any problem getting SCRs working robustly.

I have some of the same capacitors as you and have heard of high failure rates.... ~1%? I have not experienced a failure yet but have been firing mostly in the 300-340v range.

rp181

Sat Jan 03 2009, 10:06PM

Registered Member #1062 Joined: Tue Oct 16 2007, 02:01AM
Location:
Posts: 1529

Having coil heatsinks is unreasonable, as you said, the heat will not travel fast enough. What AWG wire are you using? With a couple of shots with a 1kJ bank, through a 12 AWG coil, it gets hot enough to warp tape. Having coil heatsinks is unreasonable, it will get hot nontheless. In addition to that, SCR's need a recovery time after a surge, it can range anywhere from 1 second to a minute, depending on the SCR, peak current, and di/dt. High tempeture insulation usually means more resistive and thicker insulation. If you really are worried about heat, a casing filled with oil will work. The oil will have enough thermal mass to avoid using finned heatsinks.

If your worried about eddy currents, slot the barrel. Cutting slots (mill's or dremels work well) reduces eddy currents, it is commenly done when people use brass barrels.

Having a overall view of your setup would be usefull for your wiring.

You said you where going to get power from one main bank rather then splitting it up. This doesn't make sense to me, as it would mean the first coil is more powerfull, and progressively gets weaker.

Usually people use lyctic capacitors for hobby use in expierementation. If you plan on having high shot rate design, you would be better off using pulse capacitors, or atleast new lyctics. One capacitor i ordered from that site was dry, and you have no way of knowing how old it is.

first previous 4 5 6 7 next last

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob